1. Field of the Invention
The present invention relates generally to apparatus for applying solder paste to printed circuit boards and, more particularly, to high speed apparatus which applies solder paste to printed circuit boards in a continuous operation.
2. Description of the Prior Art
In the early days of operations with printed circuit boards ("PCB's") or so called thick film circuitry, it was customary to mount components by the pin in hole method. This remains the most common method presently utilized. According to this technique the board must be punched or drilled in appropriate locations to receive the leads from the various components to be mounted. Solder is then manually introduced into each hole in order to electrically connect this component with the remainder of the circuitry on the board. This is a costly and time consuming process which substantially increases the cost of the end item being manufactured.
Other techniques have been attempted and utilized in an effort to increase the speed of the manufacturing process. Typical of these is the "drag bath" process according to which the board is dragged or pulled at low speed over the surface of the solder bath such that substantially its entire under surface touches the fluid solder. At the end of its passage over the solder, the board is removed. A typical form of apparatus for performing the drag bath process is disclosed in U.S. Pat. No. 4,090,654 to Volkert.
A similar process is referred to as "wave soldering" according to which the printed circuit board to which components are to be attached by soldering is carried over a wave of solder which is created by steady agitation of the fluid solder in a bath. A short but wide stationary wave of solder is created within the bath. A printed circuit board is passed over the wave which splashes the board as it passes over the wave, touching it at its underside, in a line, which sweeps along the whole of its underside. The advantage of this method is that a relatively small amount of heat is transferred to the printed circuit board. A disadvantage of this method is that the fluid solder which has a tendancy to oxidize, becomes oxidized more quickly in view of the steady agitation.
Relatively common throughout the industry at the present time are printing machines that screen solder paste to printed circuit boards. These are generally of the clam shell design. The clam shells open and close on a hinged bearing with the PCB positioned under the top half of the clam shell, the unit is closed.
Then, a rubber wipe slides across a stencil on which the solder paste is positioned and this paste is pushed through the stencil onto the PCB. The clam shell mechanism then opens, the PCB is removed, and the cycle is repeated. Although effective in achieving the end result, this method is a slow, discontinuous one, time consuming and, therefore, expensive.
In other industries, namely, in the textile industry and in the paper making industry, screen printing machines have become popular for printing for fabrics such as woven and knitted fabrics, fibrous webs, paper, synthetic plastic foil, carpets, rugs and the like in a continuing process. An example of an ink dispensing arrangement for use in such a machine is disclosed in U.S. Pat. No. 4,023,487 to Mitter. While the Mitter device appears to be desirable for its intended purpose, it is not designed for, nor capable of, operating with the solder paste utilized in the instance of printed circuit boards.
Examples of successful machines which exhibit the clam shell design are the DEK Model 200 manufactured by Universal Instruments Company of Binghamton, N.Y. and Model No. American Tempo 2230 manufactured by Advanced Process Supply Company of Chicago, Ill.